Before our Global Positioning System (GPS) navigation devices can tell us where we are, the satellites that make up the GPS need to know exactly where they are. For that, they rely on a network of sites that serve as “you are here” signs planted throughout the world. The catch is, the sites don’t sit still because they’re on a planet that isn’t at rest, yet modern measurements require more and more accuracy in pinpointing where “here” is.
To meet this need, NASA is helping to lead an international effort to upgrade the four systems that supply this crucial location information. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., in partnership with NASA’s Goddard Space Flight Center in Greenbelt, Md., where the next generation of laser ranging and radio interferometry systems is being developed and built, is bringing all four systems together in a state-of-the-art ground station.
This is interesting. Details here.
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The JPL’s global GPS receiver network has been around for about 20 years or so. Once upon a time you joined the network and in turn received corrections so that you could get better positioning without the GPS ‘P code’. Laser ranging work has been going on for even longer (though the size of the transmitters and receivers has shrunk a bit since the bad old days). For example, the long defunct NASA Orroral Valley Laser Ranging Station (http://www.carnarvonspace.com/wiki/index.php?title=Yatharagga:_Satellite_Laser_Ranging_%28SLR%29_Station) and a long-running nearby successor owned by the Australian government and operated by Geosciences Australia (http://www.ga.gov.au/about-us/news-media/news-2004/mount-stromlo-back-in-orbit.html).
So – instruments may be improving, but this is a very old project (or at least a successor to a very old project).
GPS was commercially available in 1992 when we sailed across the Atlantic. For $3000 or so, you could get a set that gave latitude and longitude (none of those cute charts) as long as the military wasn’t messing around with it just then. We didn’t have the spare 3 grand, so we had an old SatNav — a previous satillite navigation system. The satillites for that system were expiring and not replaced, so we could easily go four hours between good fixes. That was no problem on the days far from land, but hairraising threading between the Azores islands in the dark. I would have killed for a GPS that night.
We were still better off than two Danish sailors, who crossed with just a sextant, and no radar. They had to stand off the Azores for three days, waiting for fog to clear.
And, of course, whatever system people had, they had to cross check with charts that were often based on sextant readings from the nineteenth century. So, the satillite navigation could be spot on, but the charts a mile or more off.
It makes me laugh when people complain that GPS can be a few feet off. We’ve forgotten how lucky we are.